Organic light-emitting diodes (OLEDs)1,2,3,4,5, quantum-dot-based LEDs6,7,8,9,10, perovskite-based LEDs11,12,13 and micro-LEDs14,15 have been championed to fabricate lightweight and flexible units for next-generation displays and active lighting. Although there are already some high-end commercial products based on OLEDs, costs must decrease whilst maintaining high operational efficiencies for the technology to realise wider impact. Here we demonstrate efficient action of radical-based OLEDs16, whose emission originates from a spin doublet, rather than a singlet or triplet exciton. While the emission process is still spin-allowed in these OLEDs, the efficiency limitations imposed by triplet excitons are circumvented for doublets. Using a luminescent radical emitter, we demonstrate an OLED with maximum external quantum efficiency of 27 per cent at a wavelength of 710 nanometres—the highest reported value for deep-red and infrared LEDs. For a standard closed-shell organic semiconductor, holes and electrons occupy the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs), respectively, and recombine to form singlet or triplet excitons. Radical emitters have a singly occupied molecular orbital (SOMO) in the ground state, giving an overall spin-1/2 doublet. If—as expected on energetic grounds—both electrons and holes occupy this SOMO level, recombination returns the system to the ground state, giving no light emission. However, in our very efficient OLEDs, we achieve selective hole injection into the HOMO and electron injection to the SOMO to form the fluorescent doublet excited state with near-unity internal quantum efficiency.
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The datasets collected and analysed in this work are available at https://doi.org/10.17863/CAM.31543.
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X.A., S.D., H.G., Y.C. and F.L. are grateful for the financial support received from the National Key R&D Program of China (grant number 2016YFB0401001), the National Natural Science Foundation of China (grant numbers 51673080 and 91233113) and the National Key Basic Research and Development Program of China (973 programme, grant number 2015CB655003). E.W.E., A.J.G. and R.H.F. thank the EPSRC for funding (EP/M01083X/1, EP/M005143/1). T.J.H.H. thanks Jesus College, Cambridge for a Research Fellowship. F.L. is an academic visitor at the Cavendish Laboratory, Cambridge and is supported by the China Scholarship Council (CSC) and the Talents Cultivation Program (Jilin University, China).
Nature thanks T. Kusamoto and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 Thermal stability and electron paramagnetic resonance measurements of TTM-3NCz and TTM-3PCz.
a, Thermogravimetric analysis measurements show thermal decomposition temperatures of 362 °C (TTM-3NCz) and 367 °C (TTM-3PCz). b, EPR spectra for solid samples at room temperature. ESR, electron spin resonance; B, magnetic field.
a, c, Cyclic voltammograms of TTM-3NCz (a) and TTM-3PCz (c) in CH2Cl2. For both TTM-3NCz and TTM-3PCz, the average of the cathodic and anodic potentials gives a reduction potential of −1.1 V, first oxidation potential of +0.4 V and second oxidation potential of +0.9 V. b, d, Multi-cycle (20 cycles) cyclic voltammetry measurements of TTM-3NCz (b) and TTM-3PCz in CH2Cl2 (d). A ferrocence cation/ferrocence (Fc+/Fc) reference redox couple was used for the measurements.
Luminescence intensity (l) of TTM-3NCz and TTM solutions (10 µM, cyclohexane) as a function of time. A pulsed laser of 355 nm with an energy density of 315 kW cm−2 (pulse width, 8 ns; frequency, 10 Hz) was used under ambient conditions.
a, EQE–current density plots for five TTM-3NCz devices. The peak EQEmax values are: 25% (device 1), 27% (device 2), 20% (device 3), 24% (device 4) and 16% (device 5). The EQE at 1 mA cm−2 is: 8% (device 1), 10% (device 2), 7% (device 3), 9% (device 4) and 7% (device 5). b, Radiance–voltage plots for the same five TTM-3NCz devices. The radiance levels for EQEmax are indicated, which can be distinguished from the noise. c, Current density–voltage plots for the same five TTM-3NCz devices.
Electroluminescence spectra for TTM-3NCz devices operated at current densities of 0.006–1.6 mA cm−2. There is no current dependence.
This file contains further experimental details; Supplementary Text: including synthesis and MO diagram derivation; Supplementary Figures: 1–2) TA data, 3–4) MO diagrams and basis set, 5–6) DFT orbitals and difference density plots; and Supplementary Tables: 1) TDDFT orbital contributions, 2) red LED literature.
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Ai, X., Evans, E.W., Dong, S. et al. Efficient radical-based light-emitting diodes with doublet emission. Nature 563, 536–540 (2018). https://doi.org/10.1038/s41586-018-0695-9
- Singly Occupied Molecular Orbital (SOMO)
- Organic Light-emitting Diodes (OLEDs)
- Excited Doublet State
- Luminescent Groups
- Triplet Excitons
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